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[85.23.48.6]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-55b6316db1dsm2211386e87.40.2025.07.30.09.29.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 30 Jul 2025 09:29:26 -0700 (PDT) Message-ID: <11ee9c5e-3e74-4858-bf8d-94daf1530314@redhat.com> Date: Wed, 30 Jul 2025 19:29:25 +0300 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [v2 02/11] mm/thp: zone_device awareness in THP handling code To: Zi Yan Cc: Balbir Singh , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Karol Herbst , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Shuah Khan , David Hildenbrand , Barry Song , Baolin Wang , Ryan Roberts , Matthew Wilcox , Peter Xu , Kefeng Wang , Jane Chu , Alistair Popple , Donet Tom , Matthew Brost , Francois Dugast , Ralph Campbell References: <20250730092139.3890844-1-balbirs@nvidia.com> <20250730092139.3890844-3-balbirs@nvidia.com> <22D1AD52-F7DA-4184-85A7-0F14D2413591@nvidia.com> <9f836828-4f53-41a0-b5f7-bbcd2084086e@redhat.com> <884b9246-de7c-4536-821f-1bf35efe31c8@redhat.com> <6291D401-1A45-4203-B552-79FE26E151E4@nvidia.com> <8E2CE1DF-4C37-4690-B968-AEA180FF44A1@nvidia.com> <2308291f-3afc-44b4-bfc9-c6cf0cdd6295@redhat.com> <9FBDBFB9-8B27-459C-8047-055F90607D60@nvidia.com> Content-Language: en-US From: =?UTF-8?Q?Mika_Penttil=C3=A4?= In-Reply-To: <9FBDBFB9-8B27-459C-8047-055F90607D60@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 7/30/25 18:58, Zi Yan wrote: > On 30 Jul 2025, at 11:40, Mika Penttilä wrote: > >> On 7/30/25 18:10, Zi Yan wrote: >>> On 30 Jul 2025, at 8:49, Mika Penttilä wrote: >>> >>>> On 7/30/25 15:25, Zi Yan wrote: >>>>> On 30 Jul 2025, at 8:08, Mika Penttilä wrote: >>>>> >>>>>> On 7/30/25 14:42, Mika Penttilä wrote: >>>>>>> On 7/30/25 14:30, Zi Yan wrote: >>>>>>>> On 30 Jul 2025, at 7:27, Zi Yan wrote: >>>>>>>> >>>>>>>>> On 30 Jul 2025, at 7:16, Mika Penttilä wrote: >>>>>>>>> >>>>>>>>>> Hi, >>>>>>>>>> >>>>>>>>>> On 7/30/25 12:21, Balbir Singh wrote: >>>>>>>>>>> Make THP handling code in the mm subsystem for THP pages aware of zone >>>>>>>>>>> device pages. Although the code is designed to be generic when it comes >>>>>>>>>>> to handling splitting of pages, the code is designed to work for THP >>>>>>>>>>> page sizes corresponding to HPAGE_PMD_NR. >>>>>>>>>>> >>>>>>>>>>> Modify page_vma_mapped_walk() to return true when a zone device huge >>>>>>>>>>> entry is present, enabling try_to_migrate() and other code migration >>>>>>>>>>> paths to appropriately process the entry. page_vma_mapped_walk() will >>>>>>>>>>> return true for zone device private large folios only when >>>>>>>>>>> PVMW_THP_DEVICE_PRIVATE is passed. This is to prevent locations that are >>>>>>>>>>> not zone device private pages from having to add awareness. The key >>>>>>>>>>> callback that needs this flag is try_to_migrate_one(). The other >>>>>>>>>>> callbacks page idle, damon use it for setting young/dirty bits, which is >>>>>>>>>>> not significant when it comes to pmd level bit harvesting. >>>>>>>>>>> >>>>>>>>>>> pmd_pfn() does not work well with zone device entries, use >>>>>>>>>>> pfn_pmd_entry_to_swap() for checking and comparison as for zone device >>>>>>>>>>> entries. >>>>>>>>>>> >>>>>>>>>>> Zone device private entries when split via munmap go through pmd split, >>>>>>>>>>> but need to go through a folio split, deferred split does not work if a >>>>>>>>>>> fault is encountered because fault handling involves migration entries >>>>>>>>>>> (via folio_migrate_mapping) and the folio sizes are expected to be the >>>>>>>>>>> same there. This introduces the need to split the folio while handling >>>>>>>>>>> the pmd split. Because the folio is still mapped, but calling >>>>>>>>>>> folio_split() will cause lock recursion, the __split_unmapped_folio() >>>>>>>>>>> code is used with a new helper to wrap the code >>>>>>>>>>> split_device_private_folio(), which skips the checks around >>>>>>>>>>> folio->mapping, swapcache and the need to go through unmap and remap >>>>>>>>>>> folio. >>>>>>>>>>> >>>>>>>>>>> Cc: Karol Herbst >>>>>>>>>>> Cc: Lyude Paul >>>>>>>>>>> Cc: Danilo Krummrich >>>>>>>>>>> Cc: David Airlie >>>>>>>>>>> Cc: Simona Vetter >>>>>>>>>>> Cc: "Jérôme Glisse" >>>>>>>>>>> Cc: Shuah Khan >>>>>>>>>>> Cc: David Hildenbrand >>>>>>>>>>> Cc: Barry Song >>>>>>>>>>> Cc: Baolin Wang >>>>>>>>>>> Cc: Ryan Roberts >>>>>>>>>>> Cc: Matthew Wilcox >>>>>>>>>>> Cc: Peter Xu >>>>>>>>>>> Cc: Zi Yan >>>>>>>>>>> Cc: Kefeng Wang >>>>>>>>>>> Cc: Jane Chu >>>>>>>>>>> Cc: Alistair Popple >>>>>>>>>>> Cc: Donet Tom >>>>>>>>>>> Cc: Mika Penttilä >>>>>>>>>>> Cc: Matthew Brost >>>>>>>>>>> Cc: Francois Dugast >>>>>>>>>>> Cc: Ralph Campbell >>>>>>>>>>> >>>>>>>>>>> Signed-off-by: Matthew Brost >>>>>>>>>>> Signed-off-by: Balbir Singh >>>>>>>>>>> --- >>>>>>>>>>> include/linux/huge_mm.h | 1 + >>>>>>>>>>> include/linux/rmap.h | 2 + >>>>>>>>>>> include/linux/swapops.h | 17 +++ >>>>>>>>>>> mm/huge_memory.c | 268 +++++++++++++++++++++++++++++++++------- >>>>>>>>>>> mm/page_vma_mapped.c | 13 +- >>>>>>>>>>> mm/pgtable-generic.c | 6 + >>>>>>>>>>> mm/rmap.c | 22 +++- >>>>>>>>>>> 7 files changed, 278 insertions(+), 51 deletions(-) >>>>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>>> +/** >>>>>>>>>>> + * split_huge_device_private_folio - split a huge device private folio into >>>>>>>>>>> + * smaller pages (of order 0), currently used by migrate_device logic to >>>>>>>>>>> + * split folios for pages that are partially mapped >>>>>>>>>>> + * >>>>>>>>>>> + * @folio: the folio to split >>>>>>>>>>> + * >>>>>>>>>>> + * The caller has to hold the folio_lock and a reference via folio_get >>>>>>>>>>> + */ >>>>>>>>>>> +int split_device_private_folio(struct folio *folio) >>>>>>>>>>> +{ >>>>>>>>>>> + struct folio *end_folio = folio_next(folio); >>>>>>>>>>> + struct folio *new_folio; >>>>>>>>>>> + int ret = 0; >>>>>>>>>>> + >>>>>>>>>>> + /* >>>>>>>>>>> + * Split the folio now. In the case of device >>>>>>>>>>> + * private pages, this path is executed when >>>>>>>>>>> + * the pmd is split and since freeze is not true >>>>>>>>>>> + * it is likely the folio will be deferred_split. >>>>>>>>>>> + * >>>>>>>>>>> + * With device private pages, deferred splits of >>>>>>>>>>> + * folios should be handled here to prevent partial >>>>>>>>>>> + * unmaps from causing issues later on in migration >>>>>>>>>>> + * and fault handling flows. >>>>>>>>>>> + */ >>>>>>>>>>> + folio_ref_freeze(folio, 1 + folio_expected_ref_count(folio)); >>>>>>>>>> Why can't this freeze fail? The folio is still mapped afaics, why can't there be other references in addition to the caller? >>>>>>>>> Based on my off-list conversation with Balbir, the folio is unmapped in >>>>>>>>> CPU side but mapped in the device. folio_ref_freeeze() is not aware of >>>>>>>>> device side mapping. >>>>>>>> Maybe we should make it aware of device private mapping? So that the >>>>>>>> process mirrors CPU side folio split: 1) unmap device private mapping, >>>>>>>> 2) freeze device private folio, 3) split unmapped folio, 4) unfreeze, >>>>>>>> 5) remap device private mapping. >>>>>>> Ah ok this was about device private page obviously here, nevermind.. >>>>>> Still, isn't this reachable from split_huge_pmd() paths and folio is mapped to CPU page tables as a huge device page by one or more task? >>>>> The folio only has migration entries pointing to it. From CPU perspective, >>>>> it is not mapped. The unmap_folio() used by __folio_split() unmaps a to-be-split >>>>> folio by replacing existing page table entries with migration entries >>>>> and after that the folio is regarded as “unmapped”. >>>>> >>>>> The migration entry is an invalid CPU page table entry, so it is not a CPU >>>> split_device_private_folio() is called for device private entry, not migrate entry afaics. >>> Yes, but from CPU perspective, both device private entry and migration entry >>> are invalid CPU page table entries, so the device private folio is “unmapped” >>> at CPU side. >> Yes both are "swap entries" but there's difference, the device private ones contribute to mapcount and refcount. > Right. That confused me when I was talking to Balbir and looking at v1. > When a device private folio is processed in __folio_split(), Balbir needed to > add code to skip CPU mapping handling code. Basically device private folios are > CPU unmapped and device mapped. > > Here are my questions on device private folios: > 1. How is mapcount used for device private folios? Why is it needed from CPU > perspective? Can it be stored in a device private specific data structure? Mostly like for normal folios, for instance rmap when doing migrate. I think it would make common code more messy if not done that way but sure possible. And not consuming pfns (address space) at all would have benefits. > 2. When a device private folio is mapped on device, can someone other than > the device driver manipulate it assuming core-mm just skips device private > folios (barring the CPU access fault handling)? > > Where I am going is that can device private folios be treated as unmapped folios > by CPU and only device driver manipulates their mappings? > Yes not present by CPU but mm has bookkeeping on them. The private page has no content someone could change while in device, it's just pfn. >> Also which might confuse is that v1 of the series had only >> migrate_vma_split_pages() >> which operated only on truly unmapped (mapcount wise) folios. Which was a motivation for split_unmapped_folio().. >> Now, >> split_device_private_folio() >> operates on mapcount != 0 folios. >> >>> >>>> And it is called from split_huge_pmd() with freeze == false, not from folio split but pmd split. >>> I am not sure that is the right timing of splitting a folio. The device private >>> folio can be kept without splitting at split_huge_pmd() time. >> Yes this doesn't look quite right, and also >> + folio_ref_freeze(folio, 1 + folio_expected_ref_count(folio)); > I wonder if we need to freeze a device private folio. Can anyone other than > device driver change its refcount? Since CPU just sees it as an unmapped folio. > >> looks suspicious >> >> Maybe split_device_private_folio() tries to solve some corner case but maybe good to elaborate >> more the exact conditions, there might be a better fix. >> >>> But from CPU perspective, a device private folio has no CPU mapping, no other >>> CPU can access or manipulate the folio. It should be OK to split it. >>> >>>>> mapping, IIUC. >>>>> >>>>>>>>>>> + ret = __split_unmapped_folio(folio, 0, &folio->page, NULL, NULL, true); >>>>>>>>>> Confusing to  __split_unmapped_folio() if folio is mapped... >>>>>>>>> From driver point of view, __split_unmapped_folio() probably should be renamed >>>>>>>>> to __split_cpu_unmapped_folio(), since it is only dealing with CPU side >>>>>>>>> folio meta data for split. > > > Best Regards, > Yan, Zi > --Mika